This invention relates generally to infrared radiation detectors and methods for converting infrared radiation to a visible image. More particularly, the invention relates to a nulling circuit coupled to an infrared radiation sensor such as a micro-electromechanical device or a bolometer device formed on a semiconductor substrate. The present invention also provides for an increased linearity and dynamic range of the micro-electromechanical or bolometer device being used as an infrared radiation
Various optical detectors such as infrared radiation detectors are available in today""s electronics industry. Many techniques for converting infrared radiation to visible images, are also known. One such example of an infrared imager available in today""s art includes a deflectable microelectromechanical (MEM) cantilever device formed of a bi-material on a semiconductor substrate. The bi-material portion of the micro-cantilever device is formed of two different materials sharing a contiguous surface, and having mismatched thermal coefficients of expansion (TCE). Examples of such bi-material MEM micro-cantilever devices and methods for forming the same, are as disclosed in U.S. Pat. No. 5,844,238 issued to Sauer et al. and U.S. Pat. No. 6,140,646, issued to Lurie et al.
The bi-material MEM micro-cantilever devices presently available in the art, bend, or deflect, when infrared radiation is absorbed upon an absorber element of the micro-cantilever heating the bi-material section of the micro-cantilever, thereby urging one of the bi-materials to expand at a greater rate than the other bi-material, thereby causing the micro-cantilever to deflect, or bend. The terms bend and deflect may be used interchangeably hereinafter. An example of a micro-cantilever such as known of the art, may be seen in FIGS. 1 and 2.
FIG. 1 is a cross sectional view showing micro-cantilever 100. Micro-cantilever 100 is suspended over substrate 200. Space 140 forms the opening between substrate 200 and micro-cantilever 100. Micro-cantilever 100 includes thermal isolation region 120, bi-material region 110, and absorber region 130. Within absorbing region 130, absorber material 210 helps to absorb infrared radiation which may be incident upon the micro-cantilever structure. In bi-material region 110, first bi-material film 215 and second bi-material 220 are present. Bi-material films 215 and 220 share a contiguous surface and have mismatched thermal coefficients of expansion. Therefore, when infrared radiation is incident upon the MEMs structure, the micro-cantilever 100 is heated and bends because of the two bi-material films 215 and 220 expand at different rates. FIG. 1 shows micro-cantilever 110 in a xe2x80x9crestxe2x80x9d position, not exposed to infrared radiation.
FIG. 2 shows the micro-cantilever structure of FIG. 1 in a deflected or bent position. The distance xcex94x indicates the height of deflection of the deflectable portion of the micro-cantilever, above the rest position. Various means are available in the art to provide an optical image having an intensity which varies along with the degree of the deflection, for example xcex94x, of the micro-cantilever structure. More generally, various means are available in the art to provide an optical image having an intensity which is determined by the amount of infrared radiation sensed by an infrared sensor.
It can be seen that a drawback of the present technology lies in the fact that such a micro-cantilever structure can only bend to a certain degree. A physical limitation to the extent of bending of the micro-cantilever exists. This limits the dynamic range and the range of linearity of the device.
When infrared radiation is incident upon such a micro-cantilever being used as an optical detector, it is desired to produce a visible image having an intensity which varies directly with the intensity of the incident infrared radiation. As a micro-cantilever device bends in response to such incident infrared radiation, it approaches a physical limitation to its degree of bending. For example, if a micro-cantilever device is fabricated so as to bend downward in response to incident infrared radiation, the physical limitation is reached when the micro-cantilever touches the substrate over which it is formed. For a micro-cantilever device chosen to bend upward in response to incident infrared radiation, this, too, will reach a physical limitation point past which it can no longer bend. As such, when this point of the physical limitation of bending is approached, the micro-cantilever device is more resistant to bending and therefore, less responsive to additional infrared radiation. An increased amount of incident infrared radiation will not cause the same extent of bending as when the micro-cantilever is in the rest position. While a significantly higher dose of infrared radiation may force the micro-cantilever to bend slightly more towards its physical limitation, the degree of this bending will not be proportional, so the device will not be linear in this region. Thus, the linear range of the device is limited.
Moreover, after the physical limitation point is reached, additional incident infrared radiation will simply not cause any further bending. This limits the dynamic range of the device. Since the intensity of an optical image ultimately produced from such a device, is based on the degree of bending, it can be seen that such a device having a poor dynamic range and limited linearity, produces an image having the same shortcomings.
Various methods for sensing the degree of bending are available in the art. Examples of such methods include optically measuring the distance between the micro-cantilever and the substrate, and electrically measuring the capacitance of a capacitor which includes an electrode formed in the substrate and another electrode formed in the micro-cantilever above the substrate. Various methods for producing a visible image having an intensity based upon the extent of bending, are also known.
Methods for producing various embodiments of micro-cantilever structures are available in the art, for example, methods for forming various micro-cantilever devices are disclosed in U.S. Pat. No. 5,844,238 as above. The present art also includes various configurations of physical micro-cantilever or other MEM structures. For example, FIGS. 3 and 4 show two embodiments of MEM structures which bend in response to incident infrared radiation. Each of the structures shown in FIGS. 3 and 4, include a bendable bi-material arm 150 and an absorber area 152. It can be understood that, in addition to the structures shown in FIGS. 1-4, various other configurations for MEMs structures which bend or deflect in response to incident radiation, can be used and have been provided in the art.
The present invention addresses the shortcoming of the limited dynamic range and limited linearity of the optical detectors using bi-material MEMs structures available in the art, by providing an optical detector using nulling circuitry along with an infrared sensor, for increased linearity and increased dynamic range.
The present invention embodies both the method and apparatus for converting infrared radiation to a visible image. The present invention provides a micro-electromechanical (MEM) device formed on a semiconductor substrate. The device includes a deflectable micro-cantilever formed of a bi-material element. The deflectable micro-cantilever is suspended above the substrate and bends in response to incident infrared radiation. Nulling circuitry is also provided and coupled to the micro-cantilever. The position of the micro-cantilever, or extent of bending may be sensed using optical, capacitive, or other means. As the micro-cantilever begins to bend in response to infrared radiation, the nulling circuitry detects the bending, and provides a signal to an element capable of providing a stimulus to restore the micro-cantilever to its original position. Examples of such elements include a resistor/heater, an electrostatic element, or a piezoelectric element. The signal which is supplied to the element, tends to maintain the micro-cantilever in its original position. Therefore, the strength of the signal required to maintain the micro-cantilever at its original position varies with the degree of incident infrared radiation. The signal supplied to the element is monitored and a visible image is formed having an intensity which is proportional to the signal supplied to the element. In this matter, the linearity of the system is independent of the detector itself.
The present invention also provides a bolometer coupled to similar nulling circuitry and thermally coupled to a heater/resistor. The bolometer serves as a detector of infrared radiation. When the bolometer is exposed to incident infrared radiation and the resistivity of the bolometer begins to change, the nulling circuitry detects the changed resistivity and provides a signal to the heater/resistor to compensate for the incident radiation and to maintain the bolometer at a constant resistivity. As in the micro-cantilever example, a visible image may be formed having an intensity which is proportional to the signal supplied to the heater/resistor.